The present invention generally relates to semiconductor devices and more particularly to a resonant-tunneling transistor that is operated in response to irradiation of light.
Since the pioneering work of Esaki and Tsu (Esaki, L. and T. Tsu, "Superlattice and Negative Conductivity in Semiconductors," IBM Research Note, RC-2418, 1969), intensive efforts have been made to realize the quantum effect semiconductor devices that utilize the resonant-tunneling of carriers through a quantum well structure formed in the device. Typically, the quantum well structure is formed by a quantum well layer sandwiched between a pair of extremely thin barrier layers. In such a quantum well structure, there is formed a quantum level in the quantum well layer at an energy level corresponding to the thickness of the quantum well layer, and the quantum well structure shows a high transmittance when the energy of the incident electrons coincides with the energy level of the quantum state in the quantum well layer. Such a phenomenon is called the resonant-tunneling and is characterized by a negative differential resistance in which the current flowing through the device decreases with increasing input voltage.
For example, Esaki has proposed in the U.S. Pat. No. 280,141, a resonant-tunneling transistor that uses the negative differential resistance for the principle of operation. The concept of the transistor has been further developed by many researchers and various devices are proposed so far. For example, Yokoyama et al. (Yokoyama, N., Imamura, K., Muto, S., Hiyamizu S. and Nishi, H., "A New Functional, Resonant-tunneling Hot Electron Transistor (RHET)," Japanese J. Appl. Phys. 24, L853, 1985) proposed a resonant-tunneling hot electron transistor (RHET) wherein the flow of hot electrons that has passed through the quantum well structure is controlled in response to a control voltage applied to a base layer that is provided adjacent to the quantum well structure.
FIG. 1 shows a fundamental resonant-tunneling transistor corresponding to the device proposed by Esaki.
Referring to FIG. 1, the transistor is constructed on a semi-insulating GaAs substrate 11 and includes a layered body 10 of semiconductor materials that in turn includes an n-type collector layer 12 of GaAs provided on the substrate 11, a lower barrier layer 13 of undoped AlGaAs that is formed on the collector layer 12, an active layer 14 of undoped GaAs provided on the lower barrier layer 13, an upper barrier layer 15 of undoped AlGaAs provided on the active layer 14, and an emitter layer 16 of n-type GaAs provided on the upper barrier layer 15. There, the thickness of the active layer 14 is less than about 100 .ANG. and there is formed a quantum well structure characterized by a discrete quantum level in the active layer 14.
The layered body 10 is formed to have a mesa structure 10A such that the upper major surface of the collector layer 12 and the upper major surface of the base layer 14 are exposed at the outside of the mesa structure 10A. The mesa structure 10A in turn is covered by an insulating film 17, and an emitter electrode 18 is provided in contact with the upper major surface of the emitter layer 16 via a contact hole provided in the insulating film 17, a base electrode 19 is provided in contact with the upper major surface of the base layer 14 via a contact hole in the insulating film 17 at a part of the layered semiconductor body 10 located outside of the mesa structure, and a collector electrode 20 is provided in contact with the upper major surface of the collector layer 12 via a contact hole in the insulating film 17 at a part of the layered body 10 located outside of the mesa structure 10A. Thereby, a quantum well is formed in the base layer 14 that is bounded by the upper and lower barrier layers 15 and 13 and there is formed a quantum state in the base layer 14 at an energy level corresponding to the thickness of the layer 14.
In the device of FIG. 1, a drive voltage is applied across the collector electrode 20 and the emitter electrode 18 such that the emitter layer 16 is biased negatively with respect to the collector layer 12. Further, a variable bias voltage is applied across the collector electrode 20 and the base electrode 19. By suitably choosing the base voltage, one can set the energy level of the quantum state formed in the base layer 14 to be coincident with the energy level of the electrons in the emitter layer 16. Thereby, one observes the resonant-tunneling of the electrons from the emitter layer 16 to the collector layer 12, and this conventional device shows the negative differential resistance.
The conventional device of FIG. 1, however, has a fabrication problem because of the extremely small thickness of the base layer 14 that is typically smaller than about 100 .ANG.. It will be understood that the formation of the base electrode 19 on such a thin base layer 14 via the contact hole in the insulation film 17 without damaging the base layer 14 is extremely difficult. When the electrode 19 penetrates through the base layer 14, the base and the collector of the transistor are shorted and the device no longer operates properly. Thus, the fabrication of the device of FIG. 1 in the production line has been unsuccessful.
In order to avoid the above problem of fabrication, various structures are proposed. For example, Capasso et al. proposed a so-called quantum well base transistor wherein the double potential wall quantum well structure is formed in a part of a thick base layer for controlling the flow of electrons through the conduction band of the base layer (Capasso, F., Kiehl, R. A. "Resonant Tunneling Transistor with Quantum Well Base and High-energy Injection," J. Appl. Phys., 58 p.1366, 1985). Further, Yokoyama et al. (op. cit.) proposed the RHET device as described before.
In any of these conventional devices, the control of the flow of the carriers is made electrically by controlling the potential level of the base region via electric means. On the other hand, it is thought that the control of the quantum effect semiconductor devices via optical means would provide the possibility of using these devices in the optical processing devices or future optical computers. When such a device is realized, it will open a new field of application of the resonant-tunneling transistors.